1. Field of the Invention
The present invention generally relates to an image reading apparatus including plural line image sensors reading the same data in an overlapped reading portion and capable of joining the image data in the overlapped reading portion to read the entire image data, and a document reading apparatus and an image forming apparatus having the image reading apparatus.
2. Description of the Related Art
In an image reading apparatus, the document image width of the A0 size is typically about 840 mm. Therefore, when the whole A0 size document is read by a single CCD line image sensor under 600 dpi resolution, as many as approximately 20,000 pixel image sensors are required to be arranged on the single line image sensor. However, a line sensor having such a large number of pixels may be difficult to be manufactured with current technology at reasonable costs, namely the manufacturing cost of such a line sensor may be very high.
Therefore, in a document reading apparatus configured to be capable of reading an A0 size and/or A1 size document, there has been widely used a method of reading an image in which plural CCD line sensors are arranged partially overlapping each other in an overlapped reading portion in the main scanning direction, and image data of the overlapped reading area is joined to each other to obtain the entire image data (see, for example, Patent Documents 1 and 2).
However, in a case where this method is employed, even when the overlapped reading portion of the line sensors is strictly adjusted in its manufacturing process, the data of the overlapped reading portion may be slightly out of alignment between the adjoining CCD line sensors (i.e., image data signal detected by adjoining CCD line sensors may not be adaptively joined to each other) due to vibration during transportation, being out of alignment in its main body of the system during its mounting, or the like. Under such conditions, when an image is read, the image may be deformed because the image data in the overlapped reading portion are not adaptively joined to each other.
Further, the conjugation length of the image forming optical system of the image reading apparatus typically requires approximately 500 mm to 800 mm, and the reduction ratio of the optical system becomes approximately 1/15. Therefore, if the CCD line sensor is misaligned by 10 μm, the image on the surface of a document may be misaligned by as much as 150 μm. This misalignment may also be caused by a thermal expansion due to the temperature change in the image reading apparatus. Such misalignment (position shift) may occur in both the main scanning direction and the sub scanning direction.
To overcome the problem, Patent Document 3 discloses a technique in which a marking is formed by using an LED in the overlapped reading portion of the CCD line image sensors, and the whole image forming optical system is configured to be mechanically moved in the sub scanning direction to detect the relative position of the optical systems, and a sub scanning position is adjusted so that the position shift can be minimized.
Further, Patent Document 4 proposes a technique in which a misalignment (position shift) in the main scanning direction is detected using a correction projection line extending in the direction parallel to the sub scanning direction, and a misalignment in the sub scanning direction is detected using two projection lines not parallel to the correction projection line, and image data are joined by finely adjusting the line synchronization of the CCD driving clock.
Further, the applicant of the present invention has proposed a configuration capable of accurately detecting the position shift (misalignment) in the sub scanning direction without moving the entire optical system by enabling a reference pattern for detecting the misalignment to be moved (Japanese Patent Application No. 2007-281540).                Patent Document 1: Japanese Patent Application Publication No.: S62-101170        Patent Document 2: Japanese Patent Application Publication No.: S56-126373        Patent Document 3: Japanese Patent Application Publication No.: 2000-358140        Patent Document 3: Japanese Patent Application Publication No.: 2006-25289        
However, when such a technique as proposed in Patent Document 3 is employed, there may arise a problem that the line image sensors are required to be mechanically moved, and guide rods and cams are required to be provided to guide the line image sensors, thereby increasing the cost and impairing the durability of the image reading apparatus in response to the additional feature.
In addition, even when the image data in the overlapped reading portion are adaptively adjusted, the parallelism between the optical systems may be degraded by the position shift (misalignment) of the optical system due to a fact that the guide rods become loose and the like. Further, in order to detect the position shift, the line image sensors are required to be moved back and forth to detect the peak position of the line image sensors. Therefore, it may take time to complete the adjustment process, namely, a long wait time may be required.
In the apparatus described in Patent Document 4, when the position shift in the sub scanning direction is detected, each of the outer width of the two non-parallel lines are measured, so that the difference between those outer widths is obtained and the misalignment amount in the sub scanning direction is detected based on the difference. Because of this feature, if there is a magnification error in the optical system, the error in the position measurement increases in proportion to the amount of the magnification error.
Further, according to Patent Document 4, the center position of the pattern is detected by assuming that 50% of the peak value is the center value. Therefore, in a case where the illumination in the projection pattern is uneven, the center position may not be accurately obtained. Further, the position is calculated without using any interpolation with respect to the edge portions of the projection pattern, therefore, the position can be detected with at most one dot accuracy (see FIGS. 7 and 8 in Patent Document 4).
Further, the projection position of the correction pattern may be slightly shifted due to thermal environmental change. As a result, even if relative alignment in the overlapped reading portion is performed, the absolute positions of the sensors may also be shifted due to the shifted correction pattern.
In the configuration proposed in Japanese Patent Application No. 2007-281540, it is configured that only the optical path of the reference pattern can be moved. Therefore, the moving portion is very limited, which is advantageous from both durability and accuracy points of view. However, there still remains a mechanical moving portion, therefore, an impact resistance and a vibration resistance may be reduced.